The present invention relates to imaging and, more particularly, to an apparatus, system and method for imaging within an intravascular environment, especially of vascular walls.
Endoscopic surgery, which involves the use of an imaging device to see images of the body's internal structures, has been used for decades in many different diagnostic and surgical procedures, including gall bladder removal, tubal ligation, and knee surgery. Recently, such methods have become widely used in plastic surgery, including both cosmetic and re-constructive procedures. The use of the imaging device allows such surgery to be performed in a minimally invasive manner.
An endoscope is a rigid or flexible optical instrument which comprises a tubular probe containing a small camera head, a camera control unit, a light source and a transmission cable. The endoscope is inserted through a small incision in the body and is moved to the viewing site in order to provide an image of the object of interest. The endoscope is connected to a viewing screen which magnifies the transmitted images of the object. During surgery, the surgeon is able to view the surgical area by watching the screen while moving the tube of the endoscope through the surgical area.
Endoscopy has limited application because most current endoscopes provide only flat, two-dimensional images which are not always sufficient for the requirements of precise diagnosis or treatment. There have been many attempts in the past to overcome this limitation, particularly by providing for three dimensional imaging. For example, providing stereoscopic images of an object by using two different optical paths is disclosed in a number of patents, including U.S. Pat. Nos. 5,944,655; 5,222,477; 4,651,201; 5,191,203; 5,122,650; 5,471,237; 5,673,147; 6,139,490 and 5,603,687. Further attempts to obtain a three dimensional image are disclosed in U.S. Pat. No. 4,714,319 in which two light sources are used to give an illusion of a stereoscopic image based upon shadows and in Japan Patent No. 131,622A which discloses a method for achieving the illusion of a stereoscopic image by using two alternately illuminated light sources.
Additional image enhancement techniques are shown in U.S. Pat. Nos. 5,728,044 and 5,575,754 which disclose endoscopes that make use of an additional sensor to provide location measurements of image points and in U.S. Pat. No. 6,009,189 which provides for image acquisition from different directions using one or more cameras.
The major limiting factor associated with current endoscopes is the inability to adequately light the object for imaging. For any type of photographic imaging, there must exist adequate illumination. Thus, the typical endoscope generally includes an illumination source which disperses light throughout the field of interest. However, many interior spaces of the body are problematic to illuminate sufficiently for imaging. For example, surgery on certain internal parts of the body require that relatively large areas be illuminated simultaneously because the surgery involves numerous objects at different distances from one another. Therefore, in many surgical fields of view, the distances of objects of interest from the illumination source can easily range between 2 and 20 cm, resulting in a distance ratio of 1:10. The corresponding brightness ratio may then be 1:100, causing blinding and making the distant objects all but invisible. Accordingly, the issue of regulation of illumination levels has been approached in a number of prior art sources, including U.S. Pat. No. 4,967,269 and Japan Patent Nos. 61018915A, 4236934A, 8114755A and 8024219A.
Another reason for difficulty in endoscopic illumination is that the field of view desired is often located within an opaque medium, such as blood. This problem has not been successfully solved and therefore effective endoscopic surgery is not widely available for many heart and blood vessel diseases, which are among the main causes for morbidity and mortality in Western society today.
The pathology that is the base of most acute coronary syndromes and sudden cardiac deaths is atherosclerosis. In this process, atherosclerotic plaque, which is an active collection of immune cells and smooth muscle cells along with deposits of fats, cholesterol, cellular waste products, calcium and other substances, is accumulated in the inner lining of an artery. Such adhered and stable plaques, which cause the more significant narrowing of the arterial wall, are considered the major factor in the development of angina pectoris. However, studies from recent years have shown that angina, myocardial infarctions and sudden cardiac related deaths are often caused by unstable plaques, also known as vulnerable plaques. These unstable plaques consist mainly of unadhered particles of the same materials as the stable plaque. If released into the bloodstream, these particles may cause small occlusions in the coronary arteries and may also cause occlusions in small blood vessels of other organs, such as the brain, kidney, or lungs. Such unstable plaque is usually smaller and therefore more difficult to detect with currently used angiographic methods, particularly the minimally invasive procedures.
Angiography is a commonly used method for imaging within the heart and coronary blood vessels. Angiography involves using an X-ray camera positioned outside the body as the imaging device, and introducing a contrast substance via a catheter into the heart and associated blood vessels to enable them to be viewed by the X-ray camera. Angiography gives a two-dimensional monochromatic view of the heart and blood vessels as viewed from the outside. This method detects occlusions by identifying places where blood flow is restricted and may further be used to direct a stent delivery system to the occlusion location, including insertion of any percutaneous transluminal coronary angioplasty device or stent. However, angiography does not give a direct view of the occlusion site nor of the plaque itself. Therefore, minimally invasive surgery performed using angiography as its visual guide carries with it a major risk of merely rupturing or disrupting the fibrous cap covering the stable plaque and consequently releasing newly unstable plaque particles into the blood stream.
Another approach to imaging in blood is to utilize infrared (IR) light to enable visibility through the suspended particles and cells in the blood. A patent that discloses a method for using deep-IR light for imaging through blood is U.S. Pat. No. 6,178,346. However, the use of deep-IR wavelengths to achieve visibility in a blood medium as described therein requires very high-energy illumination, which has risks and disadvantages when used inside the body. The use of near-IR radiation substantially diminishes risks but results in a lower resolution image. U.S. Pat. No. 4,953,539 discusses the use of endoscopic imaging using IR illumination from outside the body. It is a well-known property of human tissue to have different absorption, scattering, and attenuation coefficients of IR radiation. Thus, different types of tissues may be clearly distinguished using IR imaging techniques. However, such external illumination does not produce high quality images and has not been used to date for intra-vascular imaging. A discussion of the use of IR illumination for medical imaging is found in “A Review of the Optical Properties of Biological Tissues” Cheong, Prahl and Welch, IEEE J. of Quantum Electronics, Vol. 26, 12 Dec. 1990.
Other methods currently used to provide intra-vascular imaging include integrating endoscopes with other forms of imaging such as intra-luminal ultrasound as disclosed in U.S. Pat. No. 4,869,256 and Optical Coherence Tomography, which provides a three dimensional image by performing optical measurements. Related patents include; U.S. Pat. Nos. 6,129,672; 6,099,475; 6,039,693; 6,134,003 and 6,010,449. The imaging methods disclosed also do not provide high quality, high resolution images.
Other methods for enabling visibility within opaque fluids with intra-vascular applications are disclosed in U.S. Pat. Nos. 4,576,146; 4,827,907; 4,934,339; 4,998,972; 5,010,875 and 6,178,346. None of the methods disclosed provide sufficiently high quality images. For a comprehensive discussion of light transference through blood and different mediums, see “Optical Properties of Circulating Human Blood in Wavelength Range 400-2500 nm” Andre Roggan, Journal of Biomedical Optics, January 1999.
The optical parameters and characteristics of blood that effect visibility therethrough are primarily the absorption and the scattering of light. Normal human blood consists of 55% liquid and 45% cells. The liquid fraction is known as the plasma, and is composed of 90% water and 10% proteins. The cellular fraction is usually composed of 99% erythrocytes (red blood cells) and 1% leukocytes and thrombocytes. The component of blood that has the most significant optical effect is that of the red blood cells (RBC), primarily because of their high volume, which averages 5×106 RBC per micro-liter. Because of this very high concentration, virtually all light emitted into blood is either absorbed or scattered, making photographic imaging difficult. It has been found that the absorption and scattering coefficients of light emitted into blood vary according to wavelength in such a way that either the absorption is very high or the scattering is very high, making it virtually impossible to acquire clear and useful images using light in any wavelength from 400 nm to 2500 nm.
Of the two effects, the major impediment to acquiring images through flowing blood arises from the reflectance and consequent scattering of light by the red blood cells. Such scattering makes it impossible for a light receptor to distinguish the light reflected from structures within the blood vessel (the “signal”) from the light reflected from the flowing red blood cells “noise”). No photographic imaging method to date has successfully increased the signal/noise ratio sufficiently to overcome this impediment.
Accordingly, the need for better visibility during vascular procedures is universally acknowledged. There is thus a widely recognized need for, and it would be highly advantageous to have, an intravascular imaging apparatus that provides high quality images.